1. Field of the Invention
The present invention relates generally to a diagnosis method for detecting occurrence of an abnormality in an exhaust gas recirculation control system of an internal combustion engine (hereinafter also referred to simply as an engine) for a motor vehicle. More particularly, the invention is concerned with a method of diagnosing an exhaust gas recirculation control system of the engine with high reliability and without exerting adverse influences on the operating or running (i.e., operation performance) of the engine and hence of the motor vehicle. The invention is also concerned with an apparatus for carrying out the method.
2. Description of the Related Art
In an internal combustion engine for a motor vehicle, an exhaust gas recirculation control system is generally adopted which includes an exhaust gas recirculation passage (hereinafter also referred to as the EGR passage in abbreviation) extending from an exhaust pipe of the engine to an intake pipe for recirculating a part of the exhaust gas discharged from the engine to the intake pipe so that a maximum combustion temperature of a fuel is suppressed to such extent that generation of NO.sub.x upon combustion of fuel and contained in the engine exhaust gas can be reduced. In that case, recirculation of the exhaust gas to the intake system is not constantly performed but the timing for effectuating the exhaust gas recirculation as well as the amount of recirculation is controlled in dependence on the running or operating state of the engine by correspondingly controlling the actuation of an exhaust gas recirculation control valve (hereinafter also referred to as EGR valve) installed in the EGR passage. By way of example, in a low load range of the engine where combustion of the fuel is rather at a low level and generation of NO.sub.x is correspondingly low, recirculation of the exhaust gas is interrupted. Further, it is generally practiced to control or limit the amount of recirculation of the exhaust gas so that the ratio of the amount of exhaust gas recirculated to the amount of intake air remains substantially at a predetermined value.
In such an exhaust gas recirculation control system, an erroneous operation or occurrence of a fault in the EGR valve, a modulator for controlling the EGR valve and other components of the EGR system will make it difficult or impossible to effectuate the exhaust gas recirculation control as desired. For coping with such an inconvenience, there has been proposed an approach for diagnosing the EGR valve as to occurrence of some fault by directly detecting the actuated position of the EGR valve. This approach is, however, disadvantageous in that the exhaust gas recirculation control system is complicated in structure and becomes expensive, because provisions of not only a specific detector for detecting the actuated position of the EGR valve but also of a processing unit for processing the information supplied from the position detector are required.
As a method of detecting an abnormality or fault in the exhaust gas recirculation control system which can evade the aforementioned inconveniences, there may be mentioned a method which is disclosed in Japanese Patent Laid-Open No. 63-11127 (JP-A-63-11127), according to which an EGR valve installed in an EGR passage extending from an exhaust pipe to an intake pipe is temporarily manipulated (i.e., opened or closed) during recirculation of the exhaust gas to the intake pipe in the stable state of the engine load, and a change in the suction or intake pressure brought about by the manipulation of the control valve is detected, whereon decision is made as to whether the change in the suction pressure lies within a predetermined range. On the basis of the result of this decision, it is then determined if a fault occurs in the exhaust gas recirculation system.
The principle underlying this diagnosis method is based on the fact that actuation of the EGR valve is accompanied by a corresponding change in the suction pressure. Starting from this fact, it is contemplated to detect a fault occurring in the EGR valve and associated devices by checking the change of the suction pressure which is detected when a negative pressure playing a role in the actuation of the EGR valve changes. However, according to this known fault diagnosis method, it is required that the engine load is in the stable state when the EGR valve is to be temporarily actuated (opened/closed), which incurs, however, degradation in the operation performance of the engine as well as in the gas emission performance thereof and hence degradation in drivability of the motor vehicle. More specifically, in the stable state of the engine load in which the motor vehicle runs in an intermediate or high speed range, the air-fuel ratio of the fuel mixture is generally maintained at a substantially constant value. However, when the EGR valve is operated in the stable state of the engine load, the fuel-air ratio of the mixture undergoes corresponding a variation, involving the inconveniences such as mentioned above, even when the exhaust gas recirculation flow is limited.
Besides, the known diagnosis method also suffers a problem that the diagnosis can not be effected at a desired high frequency because the stable engine load state is prerequisite for effectuation of the abnormality diagnosis. Consequently, the known diagnosis method has a room for improvements in respect to the capability of detecting an abnormality of the exhaust gas recirculation system at the earliest possible stage.
As will be appreciate from the above description, the known method and apparatus for detecting abnormality in the exhaust gas recirculation system suffers from problems such as degradation in the operation performance of the engine and difficulty in detecting an abnormality at an early stage, because it is prerequisite that the temporary actuation (opening or closing) of the EGR valve be performed in the stable or steady state of the engine load.